NX2415 查看數據表（PDF） - Microsemi Corporation

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NX2415

TWO PHASE SYNCHRONOUS PWM CONTROLLER WITH INTEGRATED FET DRIVER AND DIFFERENTIAL CURRENT SENSE

Microsemi Corporation 

NX2415

Power MOSFETs Selection

The NX2415 requires two N-Channel power

MOSFETs for each channels. The selection of

MOSFETs is based on maximum drain source voltage,

gate source voltage, maximum current rating, MOSFET

on resistance and power dissipation. The main consid-

eration is the power loss contribution of MOSFETs to

the overall converter efficiency. In this design example,

eight NTD60N02 are used. They have the following pa-

rameters:

V =25V,

DS

I

D

=62A,R

DSON

=12mΩ,QGATE

=9nC.

There are three factors causing the MOSFET power

loss:conduction loss, switching loss and gate driver loss.

Gate driver loss is the loss generated by discharg-

ing the gate capacitor and is dissipated in driver circuits.

It is proportional to frequency and is defined as:

Pgate = (QHGATE × VHGS + QLGATE × VLGS ) × FS

...(24)

where QHGATE is the high side MOSFETs gate

charge,QLGATE is the low side MOSFETs gate charge,VHGS

is the high side gate source voltage, and VLGS is

the low side gate source voltage. This power dissipation

should not exceed maximum power dissipation of the

driver device.

Conduction loss is simply defined as:

PHCON =IOUT2 × D × RDS(ON) × K

PLCON =IOUT2 × (1− D) × RDS(ON) × K

PTOTAL =PHCON + PLCON

...(25)

Where the RDS(ON) will increases as MOSFET jun-

ction temperature increases, K is RDS(ON) temperature

dependency and should be selected for the worst case.

Conduction loss should not exceed package rating or

overall system thermal budget.

Switching loss is mainly caused by crossover con-

duction at the switching transition. The total switching

loss can be approximated.

PSW

=

1

2

×

VIN

×

IOUT

× TSW

× FS

...(26)

TSW is the sum of TR and TF which can be found in

mosfet datasheet, IOUT is output current, and FS is switch-

ing frequency. Swithing loss PSW is frequency depen-

dent.

Soft Start and Enable Signal Operation

The NX2415 will start operation only after Vcc and

PVcc have reached their threshold voltages and EN and

ENBUS have been enabled. The ENBUS pin can be pro-

grammed to turn on the converter at any input voltage.

The ENBUS pin has a threshold voltage of 1.6V.

Once the converter starts, there is a soft start se-

quence of 4082 steps between 0 and Vp. The ramp rate

is determined by the switching frequency.

dVO =

VO

dt 4082 × FS

...(27)

The softstart time is calculated as followed:

Tstartup

=

4082

FS

...(28)

Layout Considerations

The layout is very important when designing high

frequency switching converters. Layout will affect noise

pickup and can cause a good design to perform with

less than expected results.

There are two sets of components considered in

the layout which are power components and small sig-

nal components. Power components usually consist of

input capacitors, high-side MOSFET, low-side MOSFET,

inductor and output capacitors. A noisy environment is

generated by the power components due to the switch-

ing power. Small signal components are connected to

sensitive pins or nodes. A multilayer layout which in-

cludes power plane, ground plane and signal plane is

recommended .

Layout guidelines:

1. First put all the power components in the top

layer connected by wide, copper filled areas. The input

capacitor, inductor, output capacitor and the MOSFETs

should be close to each other as possible. This helps to

reduce the EMI radiated by the power loop due to the

high switching currents through them.

2. Low ESR capacitor which can handle input RMS

ripple current and a high frequency decoupling ceramic

cap which usually is 1uF need to be practically touching

the drain pin of the upper MOSFET, a plane connection

is a must.

3. The output capacitors should be placed as close

Rev.4.8

18

05/06/08

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